This invention relates, in general, to high voltage semiconductor structures, and more particularly, to high voltage semiconductor structures having a floating guard ring termination that provides for relatively high breakdown voltage.
Breakdown of a planar junction device occurs when an electric field, which develops when the junction is reverse biased, surpasses a critical intensity. The critical intensity is a function of the resistivity of the material in which the junction is fabricated. Depending on device geometry, though, this critical intensity is usually surpassed at terminal voltages well below a maximum theoretical terminal voltage for the device, which is also called a parallel plane junction breakdown voltage. It is well known in the art that a certain amount of voltage is lost due to surface peak electric field between electrodes. This surface peak field results from surface charge and surface states as well as stored charge in overlying layers which distort and condense the electric field at the surface of the device.
This condensed electric field can result in the electric field intensity surpassing the critical intensity at relatively low potential between electrodes of the device. Therefore, it is desirable to have a smooth and evenly distributed electric field between the electrodes. Electric field concentration is commonly near a main junction of the device, and occurs near a surface of a planar structure. It has been found that this problem may be minimized and higher breakdown voltage may be achieved by the implementation of floating guard rings, or field rings surrounding device junctions. Although these guard rings are beneficial for achieving high breakdown voltages, design of such guard rings has been difficult.
Two basic schemes have been used to lay out multiple guard rings terminations for high voltage devices. In the first scheme the field ring spacing and width both decrease with increasing distance from the main junction. This provides a gradual extension of the depletion layer away from the main junction. The field rings further away from the junction could be made narrower because the depletion layer below them becomes progressively shallower. However, this approach did not work well where surface space charge was not precisely known, and did not provide a good technique for determining the actual field ring spacing.
A second scheme uses floating field rings of equal width and spacing but using more rings than the first method. This method produces a much finer gradation in the depletion layer at the edge of the device, while using up slightly more space due to the large number of rings. This design scheme, too, was sensitive to surface charge, although the large number of rings reduced the impact of surface charge variation when compared to the first scheme. Although each of these guard ring schemes resulted in higher breakdown voltages, neither approach resulted in a breakdown voltage which was near the parallel plane junction breakdown.
In a multiple guard ring structure each subsequent ring is at an incrementally higher potential with respect to the main PN junction. For multiple rings to be effective, each successive ring must support the incremental voltage. That is to say, punch-through breakdown voltage from ring-to-ring limits the allowable incremental voltage from ring-to-ring. Although it is desirable to space the rings close together to achieve finer gradation of the electric field, if the rings are close together the punch-through breakdown voltage between rings is low, and subsequent rings are not able to support the full incremental voltage. This situation results in lower breakdown voltage devices.
Alternatively, as the rings are spaced farther from each other to increase punch-through breakdown voltage, the field gradation increases, and more electric field concentration results. The higher field concentration again results in lower breakdown voltage. These two opposing mechanisms have limited practical planar junction devices breakdown voltages less than about 1500 volts. What is needed is a high voltage structure which allows close ring-to-ring spacing but also has high ring-to-ring punch-through breakdown voltage.
Another difficulty with previous multiple guard ring structures was the problem of where to put the first guard ring. A large space between the main PN junction and the first guard ring increased the voltage that a single guard ring could support. This design allowed a single guard ring to support several hundred volts. When more voltage was desired, however, it has been found that this large space establishes a high field concentration in the vicinity of the first ring, and even though this high field was acceptable for a few hundred volts, it kept the structure from achieving higher breakdown voltages. Previous designs merely added more guard rings beyond the first guard ring, without changing the first guard ring spacing. The additional guard rings were effective at lessening field intensity far away from the junction, but did not affect the field intensity which was established by the initial spacing of the first ring. Thus, the additional guard rings improved breakdown voltage somewhat, but breakdown eventually occurred at the first ring where the additional rings could not help.
Accordingly, it is an object of the present invention to provide a high voltage semiconductor structure capable of obtaining relatively high breakdown voltage.
It is another object of the present invention to provide a high voltage semiconductor structure having a relatively simple guard ring design.
A further object of the present invention is to provide a high voltage semiconductor structure which attains a uniform electric field between the electrodes.
Yet another object of the present invention is to provide a high voltage semiconductor structure which reduces the probability of punch-through between guard rings.